A method and an apparatus of the types mentioned at the outset are known from European published patent application 0 233 661. This provides a single-worm extruder having a fusion zone in which synthetic plastics material containing impurities is fused into a molten state. Downstream of the fusion zone is a mixing zone into which discharges an extraction gas supply pipe. This gas is supplied in a super-critical state, i.e. pressure and temperature are close to the critical point and may even be slightly above or below it. Preferably, pressure and temperature should settle down just above the critical point. The temperature is in the region of the temperature of the molten plastics material. Downstream of the mixing zone is a relieving zone or discharge zone, i.e. the molten material is delivered from the extruder at atmospheric pressure, an abrupt relief of tension taking place. As this happens, the gas containing the impurities ought to separate from the molten material. This method and this apparatus are altogether unsatisfactory in view of the purity which results and with regard to the possibility of gas recovery.
Known from GDR patent 69 099 are a method and an apparatus for the separation of solvents from hot molten synthetic plastics materials. In this case, two uniform-pitch twin-worm extruders are disposed serially. The first extruder comprises a cooling zone which is separated by a packing or a damming element from a downstream fusion zone. Immediately in front of the gasket, a spray-in aperture opens out into the cooling zone through which is sprayed hot molten synthetic plastics material containing solvent. Some of the solvent is condensed out and is drawn off on the downstream side through an extraction orifice. In the fusion zone, the synthetic plastics material which still contains solvent residues is again melted, the necessary pressure being established by a throttle disposed in the connecting pipe between the extruders. The second extruder has in turn, at its upstream end, a cooling zone which is separated by damming elements or gaskets from following heated degasification zones. The molten synthetic plastics material still containing residual solvent is sprayed into this cooling zone in the vicinity of the gasket, parts of the solvent being condensed in the same way and being drawn off through an extraction orifice on the upstream side. The synthetic plastics material is passed through the gasket into the vacuum zones where the rest of the solvent is withdrawn.
The so-called high-pressure extraction to which the present invention refers is generally known and is for example described in detail in the publication of Krupp Industrietechnik GmbH--Krupp Research Institute--114-440 d 1068520.